I'm not sure I understand the first question. Gift certificates
can be used to pay for postage. They are as good as cash for any Forum
More than one certificate can be used on a single purchase but the shopping
cart isn't set up for that, so it will take an email or phone call to arrange the payment.
A certificate can be used for more than one purchase. The remaning balance on the certificate automatically becomes a store credit after the certificate is used.
If I did not answer everything, please ask again. Thanks.